[DOCS] adding a note on python client versioning schema
Migrating NodesInfo API to use plugins instead of singular plugin

In order to be consistent (and because in 1.0 we switched from
parameter driven information to specifzing the metrics as part of the URI)
this patch moves from 'plugin' to 'plugins' in the Nodes Info API.
[DOCS] Add GitHub community river plugin
Made possible to dynamically update `discovery.zen.publish_timeout` cluster setting

`discovery.zen.publish_timeout` controls how long the master node is going to try and wait for all the nodes to respond to a cluster state publish before going ahead with the following updates in the queue (default 30s). Up until now changing the settings required restarting each node. The setting is now dynamic and can be changed through the cluster update settings api.

Closes #5063
Remove `custom_score` and `custom_boost_factor` queries

`custom_boost_factor` and `custom_score` were deprecated in `0.90.5`
and their documentation was removed already in `1.0`. This commit
removes all support for those queries since they are supercede by
`function_score`.
[Docs] Remove `custom_score` from documentation and add a migration
section.
[DOCS] Fix typo in rename_pattern in snapshot/restore documentation
Added test #5078
[DOC] Updated the Ruby gem version for Elasticsearch 0.90.x
Beefed up DeleteByQueryTests to run not just against a single document
[TEST] disabled replicas in validate query test